The present invention relates to internal combustion engines for automotive vehicles and more particularly to a spark-ignition multiple-cylinder internal combustion engine having an exhaust emission control arrangement.
For the purpose of reducing toxic combustible residues such as unburned hydrocarbons and carbon monoxide contained in the exhaust gases from automotive internal combustion engines, some modernized automotive vehicles are equipped with thermal reactors which are adapted to combust the combustible residues in the exhaust gases before the exhaust gases are discharged to the open air. To exploit the exhaust cleaning performance of the emission control devices of this nature and in an attempt to lessen not only the hydrocarbons and carbon monoxide but nitrogen oxides which are another important contributor to air pollution caused by automotive internal combustion engines, it has been proposed to have the cylinders of a multiple-cylinder internal combustion engine arranged in two groups and to supply a relatively rich air-fuel mixture to one group of cylinders (hereinafter referred to as rich-mixture cylinders) and a relatively lean air-fuel mixture to the other group of cylinders (hereinafter referred to as lean-mixture cylinders). The exhaust gases from the rich-mixture and lean-mixture cylinders are mixed together in the thermal reactor so that the toxic combustible compounds contained in higher proportion in the former than in the latter are completely oxidized with the agency of hot air which is contained in higher concentration in the latter than in the former. During idling or low-load operating condition of the engine, the temperature of the exhaust gases from the rich-mixture cylinders tends to lower so that the exhaust gases would fail to be completely oxidized in the thermal reactor unless the exhaust gases contain unburned hydrocarbons and carbon monoxide in relatively high concentrations. It is, for this reason, desirable to have the air-fuel mixture for the rich-mixture cylinders enriched during idling and low-load operating conditions of the engine. Enrichment of the mixture will also contribute to reducing formation of nitrogen oxides in the rich-mixture cylinders. As the load on the engine and the output speed of the engine are increased, the temperature of the exhaust gases rises so that exhaust gases can be sufficiently re-oxidized in the thermal reactor although the exhaust gases may contain combustible compounds in relatively low concentrations. The mixture to be supplied to the rich-mixture cylinders can thus be leaned out during medium to high load operating conditions of the engine. This will also give rise to an increase in the power output of the engine and will thus lend itself to reduction of the engine fuel consumption. The above mentioned range of the air-to-fuel ratio of the mixture to be supplied to the rich-mixture cylinders is determined with these in mind. On the other hand, the air-to-fuel ratio of the mixture to be supplied to the lean-mixture cylinders is determined in consideration of the extent to which the production of nitrogen oxides should be inhibited, the requirements for enabling the engine to smoothly operate, and the desired proportion of air to be contained in the exhaust gases emitted from the lean-mixture cylinders. From the purely theoretical point of view, the exhaust gases of an automotive internal combustion engine must be satisfactorily cleaned out if the engine is arranged and conditioned to bring the above described schemes to a successful issue. Actually, however, numerous operational parameters are involved in and related to the exhaust emission control performance of the internal combustion engine and, to take full advantage of the internal combustion engine of the described character, it is important that various emission control arrangements be made in the air-intake, mixture induction, exhaust and ignition systems of the engine and organically consolidated into a comprehensive system.